Which PCB Connectors Are Best?

There are many approaches to flexible (i.e., versatile) connections, but it appears that European offerings are slowly taking over the market.

At the outset of this blog, let me make full disclosure. I used to work for one of the companies that I mention in this article. Also, my current employer was spun off from, and still has a business arrangement with, that company. This arrangement has kept me occupied for 22 years. As a result, I have greater access to Weidmuller versions of the terminals I discuss than to others, so this blog is likely to appear biased in that regard. Although I will try and stay balanced, it must be said that the contents of this (and all my other blogs) are my opinions alone, and not those of my employers -- current or previous.

It seems to me that the connection requirements for industrial electronics are different than the other market sectors, with the possible exception of hobbyist electronics. In the industrial market, integrators take a bunch of functional modules and combine them with Programmable Logic Controllers (PLCs) and sensors to make a complete system whose various modules are connected by long lengths of cables. Often, some portions of the system are installed long before the others and they have to be hooked up on site. Changes in specifications and plant upgrades also mean that there must be some flexibility in the interconnections.

Industrial products do not approach the high volumes of the automotive and consumer market places and are much less cost-sensitive, trading this for interface flexibility. Mostly, the whole industrial package is concentrated and housed in cabinets. They are typically mounted on rails, and the European TS35 and TS32 rails seem to me (sitting on the fringes of the market) to be the most dominant. There are many variations of terminal blocks that mount on these rails (I may write another blog on those in the future).

There are many approaches to flexible (by which I mean versatile) connections, but it seems to me the European offerings appear to be slowly taking over the market, despite established approaches like barrier strips in the North American industry. There are also other common approaches, such as the 0.1" or 0.156" headers made by Molex, Amp, and many others which require crimp tools. Then there are spade terminations and their ilk, and you may even remember the low-end Fahnestock clips.

The first issue in making field connections is the service personnel typically have only three tools, a BFS, a BFH, and a BFW (a screwdriver, a hammer and a wrench -- the "B" stands for "big" and I leave the "F" to your imagination). The bigger the connectors used, the happier the installers are. The second issue is terminating the wire, which may also require specialized crimping tools. This is part of what the nascent industry (in the 1950s and 60s) was addressing, but -- as we shall see -- things have come full circle.

One of the problems is getting high density connections off a board and connected to the field wiring. An approach was to bring out connections from the PCB via a standard high density cable, and to provide interposing external terminals to connect to as you can see in Figure 1.

Figure 1. A 37-way D-subminiature to screw terminal adapter. The connectors are double stacked to improve the density, but are not pluggable.(Click here to see a larger image.)

Variations also allow for industry standard flat IDC (insulation displacement connector) cable to screw terminal. This kind of cable can be inflexible; it only comes in fixed lengths, and it needs specialized crimping/soldering equipment. Since it mounts horizontally on the rail, it also uses up rail space, which is often an issue as a result of under-design. The biggest issue is the isolation and current capabilities of the cables.

From my biased perch, the lion's share of the PCB screw terminal market appears to be owned by two German companies, which are, in no particular order, Weidmuller (Weidmueller in the USA) and Phoenix Contact (if you disagree, please feel free to contradict me in the comments below). There are a few more German companies (Wago and Weco, for example) and some US and Asian (try Degson Electronics) manufacturers as well.

It used to be easy to tell who made a terminal by its color. Weidmuller's offerings were orange or black, while and Phoenix's were green. More recently, Phoenix has introduced some in black, and there are now many mimics of the original shades. I should also mention that there are some higher-end housings that include connectors that are integral to the package as you can see in Figure 2.

Figure 2. A housing with specialized pluggable connectors.(Click here to see a larger image.)

There is an ever-expanding range of PCB terminals. They started out with non-pluggable screw terminals (see Figure 1 and Figure 3), and today they can be purchased with different numbers of poles. Furthermore, some can even be joined together to make terminal blocks of any length.

Figure 3. Non-pluggable screw terminals; note the ability to add clip-in tags to identify each pole of the connector.(Click here to see a larger image.)

The use of fixed terminals means that -- when removing and re-installing a module -- you have to individually unscrew every terminal and then remember how to reconnect the wires. As a result, the trend is to pluggable connectors, but there are exceptions, especially when you are using connectors for very high currents like those in Figure 4.

Your choice of which pluggable connector to use is governed by the voltage and current of the signal. Often, connectors are interchangeable between manufacturers, sometimes down to the plug of one manufacturer fitting the socket of another. The highest capacity pluggable connector I know (Figure 5) is rated at 54 amps, but I don't imagine many of you are involved in that ballpark, so I will quickly move on, not the least that it also exceeds my comfort level by many orders of magnitude.

The modern connectors we use today are heaps better than the old "Chocolate strip" screw connector blocks we used to use (and which are still around):

Although they are very handy, the fact that the screw impinges directly on the wire usually destroys the wire if you pur any more torque on it than is absolutely necessary. Even ferrules can get destroyed, though they help a lot. The modern terminal blocks which bring two flat surfaces together to hold the wire are much better.

The electrical simplicity of the connector belies the complex mechanical design needed. The contact itself is a tradeoff between mechanical strength, conductivity, clearances, corrosion resistance, gas tight, surface area, regulatory requirements, pressure and vibration resistance. The last item is often covered by a patent. Take a look at the 13th page of this document in the section marked "The principle of vibration resistance".

Pluggable connectors have a specified retention force, required by standards authorities to be as stornog as possible, yet low enough so that the user can separate them wioth reasonable ease.

Did you know that every screw terminal has a torque rating to meet the specifications? Some manufacturers actally sell torque screwdrivers and bits sutyiable for their terminals. Here is one for instance. Page F.8 for the mechanical ones and F.4 for the electric one.

I highly reccomend using the electric screwdriver when you have a lot of connections, for obvious reasons.

Most terminal manufacturers ship with the wire clamps fully open for customer convenience.

When you unscrew a terminal to remove a wire or add an additional one, you shopuld unscrew it completely (depending on you terminal's design). With the clamping mechanism in an intermediate position it is possible to insert the wire outside the clamp leading to much frustartion when you tighten the clamp only to discover the the wire just falls out and you have top repeat the process.

This is GREAT!. Don't you just love this forum- pose a question and someone has the answer.

This does lead to additional issues though- if you are using gold plated terminals then you probably should not be using ferrules which are tin plated. I have also seen wire tinned with solder which includes tin and so should not be used with gold. And I have also seen components like resistors inserted into terminals- since they are also tin plated, this would also be a no-no.

Your information is otherwise so complete that I have only one question: why is Weidmuller spelled differently in the US?

The reason goes something like this. At its source Weidmuller actually has an umlaut (the double dot) above the "u". This converts the sound of the "u" from an "uh" (as in mull) to a "ue" (as in "avenue"). So the spelling was changed to reflect the correct pronunciation.

However it didn't adress some other issues. An American today typically pronounces it Weedmewler whereas in German it is Veedmueller- but not attempt has been made to address that.

It was actually called Klippon in the UK, South Africa and Australia and Conexel in Brazil- all now changed to Weidmuller.

Aubrey, as always it is a pleasure to read something like this written by someone who knows his subject inside-out.

Flattery works! Now what was it that you wanted? LOL

Poorly soldered connectors like these and power sockets are a prime cause of failures -

Excellent point and there is a knock on implication which I saw on my own air conditioning unit. The poor solder joint lead to higher resistance, which led to overheating, which ended up burning the PCB. Still under warranty, so no worries!

Aubrey, as always it is a pleasure to read something like this written by someone who knows his subject inside-out. The only thing I can add is that when using connectors like this it is important to make sure they are soldered properly. Poorly soldered connectors like these and power sockets are a prime cause of failures - ANYTHING that has any mechanical stress at all needs a good meaty solder joint. You did allude to this in your 2nd-last paragraph. The soldering machines never seem to get it right - and I seem to get most of the resultant failures.

Your information is otherwise so complete that I have only one question: why is Weidmuller spelled differently in the US?

This is a touchy issue with all connector manufacturers. Try and get data on the number of make/break operations and how the connector degrades. As near as I can tell any pluggable connector is only rated to make/break in the tens of operations and then begin to degrade. (I have always been wary of this on devices like in-circuit programmers using flat cable connectors.) Obviously the most noticeable change would be an increase in the contact resistance. However from practical experience this number is much, much higher- hundreds, maybe even thousands and that is with light current.